Split `common` into multiple files

[apdavison]

The common module contains too many conceptually-different things. "common" can refer both to classes/functions that are independent of a particular simulator (for example, the Space class), and to classes/functions that provide a common implementation of parts of the simulator-dependent elements of the API (for example the Population class, which should be subclassed by each backend module). The former should be used by all backends. The latter (common implementation) may be used for convenience, but a backend could implement the API without having recourse to any of the common implementation.

I propose:

• to move Error classes into an errors module
• to move the Space class into a space module
• to move the standard model machinery into a standardmodels module

The various utility functions could perhaps be moved into a core module.

[apdavison]

the errors part was done in r711

[apdavison]

this is pretty much done. Possibly is_conductance(), check_weight() and check_delay() should still go, but the rest definitely belongs in common.

[apdavison]

Good enough.